The present invention concerns a uniform illumination device for the dial of a display device as a result of the particular configuration of a guide for the light emitted by at least one light source, the guide and the light source being situated at the periphery of the dial in the space substantially between the latter and a sealing crystal.
Display devices in which a dial and in particular the dial of a timepiece is illuminated by means of at least one light source, formed of a miniature electric light bulb or a diode, associated with a light guide made of translucent material in the shape of a loop, arranged between the crystal and the dial are already known. Such a device corresponds for example to that disclosed in Swiss Patent No 549 237. During use it has however been noted that the zone of the dial situated in the proximity of the light source was naturally lighter than the diametrically opposite zone. In order to overcome this drawback, U.S. Pat. No. 4,908,739 proposes to distribute at least two light sources at equal distance within an annular guide. Such a device increases the illumination of the dial locally, but does not assist in making it more uniform. By increasing the number of light sources, it would be possible to obtain more uniform illumination, but at the cost of an increase in the consumption of energy, which would constitute a critical factor for the life span of the batteries in the case of application to a timepiece.
Various other arrangements have been proposed for orienting the light coming from the guide more efficiently towards the dial. By way of example, the device of Swiss Patent No 677 306 may be cited, in which a ring forming a diffuser has a cross-section designed so that it has a polished face inclined towards the dial, the other faces being uniformly coated with a matt paint. In Swiss Patent No 336 763 a greater concentration of the light rays towards the dial is obtained by increasing the reflection of the light rays onto the crystal, whose face oriented towards the dial is coated with a thin reflective metallic layer.
As a result of these devices of the prior art, a more intense illumination of the dial is achieved, but uniform illumination is not always obtained, which is unsatisfactory from the aesthetic point of view, and as regards proper legibility of the information inscribed on the dial, such as hour-symbols in the case of an analog display timepiece.